This invention relates to an improved electrolytic process for the preparation of ethane-1,1,2,2-tetracarboxylate esters from malonate esters. The products are useful as precursors of the corresponding salts and free acids, and hydrates thereof, which are useful sequestrants and detergency builders.
Electrolytic dehydrodimeric coupling of diethyl malonate to yield tetraethyl ethane-1,1,2,2-tetracarboxylate has previously been reported in Okubo et al, Technical Reports of Osaka University, 13, 495 (1963) and Okubo et al, Bulletin of the Chemical Society of Japan, 37, 1794 (1964). In the former reference, the electrolysis was conducted on the sodium salt of diethyl malonate at 70.degree. C but in the absence of iodide ion, while in the latter reference, the electrolysis was conducted on the free diethyl malonate at sub-ambient temperatures, for example, 10.degree. C, in the presence of iodide ion electrolyte. The results, however, were less than satisfactory. For example, the current efficiency was 24 percent and 36 percent, respectively, with ethanol and acetonitrile as the solvent. In addition, the maximum percentage yield of product obtained was only 49 percent (in acetonitrile). Moreover, with ethanol as the solvent, the tetraethyl ethane-1,1,2,2-tetracarboxylate could not be isolated as a crystalline product, apparently due to the presence of numerous contaminating by-products.
It has now been discovered that the difficulties and disadvantages of the prior art methods are overcome by the process of the present invention wherein the electrolysis is conducted in the presence of iodide ion electrolyte and at elevated temperatures which represents a substantial improvement in the sense that:
(A) INCREASED CURRENT EFFICIENCY IN EXCESS OF ABOUT 65% IS OBTAINED;
(B) INCREASED PERCENTAGE YIELD OF THE DESIRED ETHANE-1,1,2,2-TETRACARBOXYLATE ESTER PRODUCTS UP TO ABOUT 98 PERCENT (BASED ON THE CONVERSION OF THE MALONATE ESTER STARTING MATERIAL) ARE OBTAINED; AND
(C) INCREASED PRODUCT PURITY IS ACHIEVED.
Various other advantages of this invention will become apparent from the accompanying description and claims.